1. Field of Invention
This invention relates to systems and methods for generating binary halftone dots.
2. Description of Related Art
Digital halftoning has evolved as a method of rendering the illusion of continuous tone, or “contone”, images using devices that are capable of producing only binary picture elements. Conventional halftoning adds a two-dimensional, spatially-periodic, dot screen or line screen structure to the images to be halftoned. Typically, the same screen, or at least a number of more or less identical screens, are used to halftone each of the color separation layers of a polychromatic, i.e., color, image. The halftone screens are oriented at different angles for printing the respective halftone color separation layers.
Misregistration between the color separation layers in a halftoned color image causes a number of problems, including gaps and overlaps between colors, color shifts and moiré. Ideally, perfect registration should be obtained. However, obtaining perfect registration is usually too difficult and/or too costly to obtain using mechanical systems.
Trapping is commonly used to reduce gaps. Using rotated halftone screens can reduce color shifts and moiré, because such rotated halftone screens are less sensitive to misregistration. However, rotated screens do not provide as large a color gamut or as fine a screen structure as that which can be achieved using a dot-off-dot halftone. Unfortunately, dot-off-dot halftones are extremely sensitive to misregistration.
Recently, systems and methods that shift or warp the image data have been developed to compensate for misregistration. These systems and methods place the image in the correct location and can avoid trapping problems. However, these systems and methods do not shift the halftone and they do not solve color shift problems that can arise from misregistered halftoned color separation layers.
The halftoning methods disclosed in U.S. Pat. No. 5,410,414 to Curry, incorporated herein by reference in its entirety, and U.S. Pat. No. 4,537,470 to Schoppmeyer, warp, i.e., adjust or move, the image data produced by an image data generator to improve registration. Such image data generators include color or gray scale image generators and binary image generators.
In many image-forming devices, a stimulus is scanned relative to the surface on which the image is formed at high rate in one direction, and a lower rate in a generally orthogonal direction. For example, in an image forming device that uses a raster output scanner, such as a laser beam, the beam is scanned across a photoreceptor along a first direction, known as the high-addressability direction, that is generally orthogonal to a process direction in which the image surface moves past the raster output scanner. Subsequent scans of the laser beam are offset from each other in a low-addressability direction that corresponds to the process direction. Thus, the low-addressability direction is generally orthogonal to the high-addressability direction. In contrast, in an image forming device that uses a page-width wide bar of LEDs or laser diodes, the high-addressability and process directions are parallel to each other, while the low-addressability direction is generally orthogonal to both the high-addressability and process directions.
High addressability or hyperacuity refers to the ability to locate an edge, occurring between one portion of an image and another portion of an image, at a resolution that is greater than the resolution of the stimulus used to form the image. Such edges often occur between halftone dots and the non-image background regions of each of the color separation layers. One common stimulus used by various image forming apparatus to form images is the light beam scanned by the raster output scanner (ROS). In various known high-addressability systems, the light beam is modulated at a rate that is four times or eight times the period it takes the raster output scanner to move the one or more light beams a distance along the high-addressability direction that is equal to the diameter of the light beam. This is known as 4× or 8× high addressability.